Re: Hi-res picture packer?

[Andy McFadden <fadden@fadden.com>]

gnudutch <gnudutch@gmail.com> wrote:
> I'm looking for a program that I used in the past for compressing hi-
> res pictures.

Just for fun I tried applying LZSS to hi-res images.  I compared the
results to the "axe packer" from the posted disk images, which appears
to do the typical RLE on alternate bytes.

It did pretty much what you'd expect: better on "complex" images, not
as well on "simple" images.  Anything with a solid color background and
relatively little activity does well with RLE, and the relatively short
match lengths in LZSS (max 18) can't compete.  I tried increasing the
match length while decreasing the maximum match distance to keep the total
encoding at 16 bits, which helped for some images but not others.

Interestingly, splitting the image into odd/even sections and compressing
them produced poor results.  I think splitting reduces the lengths
of each run, and if you compress the halves individually you're
restarting the compressor halfway through.

I suspect performing a first pass with 1- or 2-byte RLE and a second with
LZSS would work pretty well.  The LZSS decoder is very simple, making it
well-suited for this application.

Another possibility is LZB, which can use variable-length matches to
great effect, but the decoder is not so simple.

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